1. Field of the Invention
The present invention relates to an improvement in an electromagnetic clutch.
2. Prior Art
An exemplary conventional electromagnetic clutch is constructed as shown by FIG. 1, which is a sectional view taken by a sectional plane including an axis of the clutch wherein a ring-shaped armature 7', which also serves as moving clutch member, is resiliently held by a spring member 5 on a flange shaped part 41 of an output member or driven member 4, which is to be connected to a shaft of a load machine. A connection between the spring member 5 and the armature as well as a connection between the spring member 5 and the driven member 4 are provided by rivets 6 and 69. A revolving yoke 1 formed in a ring-shape and having a driving member, for example, a belt pulley 11 fixed integrally to the peripheral face thereof is rotatably journaled by a ball bearing 3, which is fixed, for example, onto a frame of the load machine (not shown). An electromagnet 2 of a ring-shape is held in a stationary manner on a frame 21, which is fixed, for example, to the frame of the load machine by bolts or the like means. The electromagnet 2 is disposed in the revolving yoke 1 in a magnetic coupling manner, so that magnetic flux .phi. is induced as shown by chain arrow lines even when the revolving yoke 1 is revolving. The magnetic flux .phi. flows through a path from an electromagnet yoke 22, through inner or outer cylindrical yoke part 12 or 13, zigzag magnetic path formed by the armature 7' with a ring-shaped groove and the touching part 101 with two concentric ring-shaped grooves 102 and 103, the outer or inner cylindrical yoke part 13 or 12, and to the electromagnet yoke 22. The above-mentioned single disk type conventional clutch of FIG. 1 has the following problems:
(1) Because the single clutch disk 7 of a large area instantly touches the touching part 101 of the driving member 1, after first contact of the clutch disk 7 to the touching part 101 the clutch disk 7 makes undesirable chattering (repetitions of many times of contacts and departs) and pulsive or vibrating slips, thereby making transmitted torque and transmitted revolution speed irregularly and pulsively increase and making a strong shock (sharp peak Tp of torque) as shown by FIG. 2. This is caused by a contact by single disk that causes instantaneous bitings (minute parts' melting by friction heat leading to bonding of clutch disk 7 and the touching part 101), each followed by departings by a large force between the above-mentioned two parts due to difference of revolving speeds between the two parts at the initial stage. Such phenomena are very much likely to occur when the load machine is a pulsive load such as refrigerant compressor or the like. It was believed for such single disk type clutch, that a rapid motion is desirable since it shortens the time period of the chattering action of the clutch which accelerate wearing of the clutch disk 7 and the touching part 101. Therefore, the designing of the clutch was to increase coupling speed, that is, to shorten the time period of coupling action. But the problem was that such rapid motion induces sharp peak of torque, namely a strong mechanical shock to the system as designated by Tp in FIG. 2, which is harmful not only for clutch itself, but also for the load machine, and often also for the motor. PA0 (2) In order to reduce the shock at the instance of clutch coupling, a torque limiting means may be used. But such means naturally decreases maximum torque to be transmitted; and furthermore, it is not effective for absorption of shocks produced by a condition of a combination of a considerable moment of inertia of the load and a considerable difference of revolving speeds between the clutch disk 7 and the touching part 101.